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trunk/workshop-foss4g/joins_exercises.rst
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Section 13: Spatial Joins Exercises
Here's a reminder of some of the functions we have seen. Hint: they should be useful for the exercises!
:command:`sum(expression)`: aggregate to return a sum for a set of records
:command:`count(expression)`: aggregate to return the size of a set of records
:command:`ST_Area(geometry)` returns the area of the polygons
:command:`ST_AsText(geometry)` returns WKT text
:command:`ST_Contains(geometry A, geometry B)` returns the true if geometry A contains geometry B
:command:`ST_Distance(geometry A, geometry B)` returns the minimum distance between geometry A and geometry B
:command:`ST_DWithin(geometry A, geometry B, radius)` returns the true if geometry A is radius distance or less from geometry B
:command:`ST_GeomFromText(text)` returns geometry
:command:`ST_Intersects(geometry A, geometry B)` returns the true if geometry A intersects geometry B
:command:`ST_Length(linestring)` returns the length of the linestring
:command:`ST_Touches(geometry A, geometry B)` returns the true if the boundary of geometry A touches geometry B
:command:`ST_Within(geometry A, geometry B)` returns the true if geometry A is within geometry B
Also remember the tables we have available:
- nyc_census_blocks
- name, popn_total, boroname, the_geom
- nyc_streets
- name, type, the_geom
- nyc_subway_stations
- name, routes, the_geom
- nyc_neighborhoods
- name, boroname, the_geom
Exercises
"What subway station is in 'Little Italy'? What subway route is it on?"
SELECT s.name, s.routes FROM nyc_subway_stations AS s JOIN nyc_neighborhoods AS n ON ST_Contains(n.the_geom, s.the_geom) WHERE n.name = 'Little Italy';name | routes -----------+-------- Spring St | 6"What are all the neighborhoods served by the 6-train?" (Hint: The routes column in the nyc_subway_stations table has values like 'B,D,6,V' and 'C,6')
SELECT DISTINCT n.name, n.boroname FROM nyc_subway_stations AS s JOIN nyc_neighborhoods AS n ON ST_Contains(n.the_geom, s.the_geom) WHERE strpos(s.routes,'6') > 0;name | boroname --------------------+----------- Midtown | Manhattan Hunts Point | The Bronx Gramercy | Manhattan Little Italy | Manhattan Financial District | Manhattan South Bronx | The Bronx Yorkville | Manhattan Murray Hill | Manhattan Mott Haven | The Bronx Upper East Side | Manhattan Chinatown | Manhattan East Harlem | Manhattan Greenwich Village | Manhattan Parkchester | The Bronx Soundview | The BronxNote
We used the DISTINCT keyword to remove duplicate values from our result set where there were more than one subway station in a neighborhood.
"After 9/11, the 'Battery Park' neighborhood was off limits for several days. How many people had to be evacuated?"
SELECT Sum(popn_total) FROM nyc_neighborhoods AS n JOIN nyc_census_blocks AS c ON ST_Intersects(n.the_geom, c.the_geom) WHERE n.name = 'Battery Park';9928"What are the population density (people / km^2) of the 'Upper West Side' and 'Upper East Side'?" (Hint: There are 1000000 m^2 in one km^2.)
SELECT n.name, Sum(c.popn_total) / (ST_Area(n.the_geom) / 1000000.0) AS popn_per_sqkm FROM nyc_census_blocks AS c JOIN nyc_neighborhoods AS n ON ST_Intersects(c.the_geom, n.the_geom) WHERE n.name = 'Upper West Side' OR n.name = 'Upper East Side' GROUP BY n.name, n.the_geom;name | popn_per_sqkm -----------------+------------------ Upper East Side | 47943.3590089405 Upper West Side | 39729.5779474286